Add optional dependency load

[danschultzer]

I'm not 100% sure if this is the best way to handle the optional dependency. It seems to fail in umbrella setups.

Also, there'll be some warnings like the following:

warning: function Mix.Tasks.Pow.Ecto.Install.run/1 is undefined (module Mix.Tasks.Pow.Ecto.Install is not available)
  lib/mix/tasks/pow.install.ex:35

warning: function Mix.Tasks.Pow.Phoenix.Install.run/1 is undefined (module Mix.Tasks.Pow.Phoenix.Install is not available)
  lib/mix/tasks/pow.install.ex:41

warning: function Pow.Ecto.Context.authenticate/2 is undefined (module Pow.Ecto.Context is not available)
  lib/pow/operations.ex:22

warning: function Pow.Ecto.Context.create/2 is undefined (module Pow.Ecto.Context is not available)
  lib/pow/operations.ex:30

warning: function Pow.Ecto.Context.delete/2 is undefined (module Pow.Ecto.Context is not available)
  lib/pow/operations.ex:46

warning: function Pow.Ecto.Context.get_by/2 is undefined (module Pow.Ecto.Context is not available)
  lib/pow/operations.ex:54

warning: function Pow.Ecto.Context.update/3 is undefined (module Pow.Ecto.Context is not available)
  lib/pow/operations.ex:38

I'll clean this up once I've decided if this PR is the right way to handle optional deps.